This invention relates to the construction of tuneable optical filters, and finds particular, but not necessarily exclusive, application in the provision of filters for use in wavelength division multiplexed (WDM) optical transmission systems.
One known form of tuneable filter employs a bulk-optics format of Fabry Pxc3xa9rot etalon in which wavelength tuning is effected by mechanically controlling the magnitude of the optical cavity length, for instance by the use of one or more piezo-electric elements. For use in optical waveguide transmission applications, a disadvantage of the use this form of filter is that it requires conversion of the light to be filtered from waveguided format into free-space format, and back again into waveguided format. Additionally there are problems in the making of suitable multilayer interference stacks for the optical cavity defining reflectors of the etalon, and in managing the tuning of that etalon.
Another form of tuneable filter is constituted by a Bragg reflector formed by irradiating, with a fringe pattern of UV light, a length of single mode doped silica optical fibre, and then adjusting the effective pitch of that grating by subjecting the fibre to tensile strain of adjustable magnitude. Drawbacks associated with this approach as currently practised include the fact that the magnitude of the photorefractive effect in such optical fibre typically limits the coupling strength (xcexa) of the resulting grating to not more than about 1 to 2 mmxe2x88x921, and such small values of the coupling strength (xcexa) produce inconveniently slow roll-off at the edges of the reflection waveband.
Additionally, there are problems associated with creep in the fibre and the structure employed to strain it. Moreover the tuning range is limited by the amount of strain to which the fibre may be safely exposed.
A further form of tuneable filter has been proposed, this being constituted by a Bragg reflector formed by irradiating, with a fringe pattern of UV light, a length of single mode polymer optical waveguide, and providing that waveguide with a Joule heater, by operation of which the effective pitch of the grating is of adjustable magnitude through the agency of the enhanced value of the thermal coefficient of refractive index of the polymer compared with that of a silica optical fibre waveguide. A somewhat larger value of the coupling strength (xcexa) should be capable of being achieved in the polymer waveguide than is achieved in the doped silica fibre waveguide, but the silica waveguide has the potential advantage of better mechanical stability than polymer. Moreover there is a not entirely insubstantial mass of polymer to be Joule heated in the case of the polymer tuneable filter, and this militates against speed of response.
The present invention is directed to a construction of a waveguide format of tuneable optical filter that is capable of a reasonably fast response.
According to the present invention, there is provided a waveguide format tuneable optical filter which filter includes first and second Bragg grating reflectors formed in an optical waveguide supported upon a substrate, which reflectors define between them a Fabry Pxc3xa9rot optical cavity that includes a phase adjuster formed by at least one slot interrupting the waveguide and containing a controllable refractive index medium.
The invention is applicable not only to single Fabry Pxc3xa9rot tuneable filters having only two reflectors defining between them a single optical cavity, but also to compound Fabry Pxc3xa9rot tuneable filters having more than two reflectors defining between them more than one optical cavity, analogous to certain forms of multi-cavity designs of dielectric interference stack filters.
Preferred forms of controllable refractive index media include media with relatively high temperature coefficients of refractive index and media whose refractive index is directly addressable by means of an applied electric field.
The fine structure in the waveguide core that defines the Bragg gratings may be created by selective exposure to UV light, but other methods providing easier access to the manufacture of gratings with larger values of the coupling strength (xcexa) will generally be preferred. One such method involves employing ion implantation to create the fine structure, implanting for instance ions of phosphorus. Preferably this ion implantation is performed after only part of the material that is to form the waveguide core has been deposited, the rest of the material being deposited after the ion implantation. In this way, the regions of raised refractive index produced by the localised ion implantation may be buried within the core material to lie substantially symmetrically about the core axis. (This ion implantation method is described in greater detail in the specification entitled, xe2x80x98Optical Waveguide Bragg Reflection Gratingsxe2x80x99, U.S. patent application Ser. No. 08/896,092; filed Jul. 17, 1997.) Another method of producing the fine structure also involves interrupting the deposition of the core material, in this instance interrupting it to allow the deposition and lithographic patterning of a thin layer of material having a higher refractive index than that of the core material. The small thickness of this intermediate layer, in comparison with that of the rest of the core, means that it is more readily patterned with the high resolution detail necessary for providing the requisite Bragg grating structure.